Sapphire R9 270X corrupts screen, but only at 1680x1050 on Digimate

Bit of a strange one - and the workaround is even stranger.

Got a Windows 7 PC built around an Asus P5Q Pro motherboard with Intel Q6600 quad core - nice bit of kit, bit dated. It ran an AMD Radeon 4570 X2 (single-handedly responsible for a huge energy bill !) on a Digimate L2262WD. Max resolution 1680x1050. Always worked fine. Anyway the Radeon 4570 finally died.

So we replaced with a good value Sapphire R9 270X 2GB GDDR5 model. Easy install, latest drivers (14.3 then 14.4). However we noticed the screen corrupts, more so when the mouse moves. You kind of get flashing blocks, mostly across the top third of the screen. It only does this at 1680x1050. Lower resolution is ok and higher is too, interestingly. Running at 1920x1080 looks a little fuzzy from the scaling but doesn't corrupt - which is strange because it almost looks like a bandwidth/timing issue - and you'd expect it to be worse at higher resolution.

• Same problem with all connector types (HDMI, DVI, VGA) and different cables.
• Clean install of Windows on a spare PC with the 270X and Digimate monitor - same problem.
• Plugged into a 42" LG TV and all fine.
• Borrowed a Dell 20" monitor which has max 1680x1050, worked fine.
• Plugged a Radeon 5450 instead of 270X, all fine

So its something to do with the monitor and graphics card combo. This was where I spotted something strange - connecting a second monitor clears the problem up. Doesn't matter which monitor is main or which combination of cables and connectors you use, as long as there is a second monitor present, then the Digimate works perfectly at 1680x1050 off the 270X.

Sapphire still haven't got back to me with any useful suggestions, although I suspect its in their firmware somewhere. Current version is 015.042.000.001